THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY. 297 



n^ire 5. G is the lid of tlie box, which is kept closed, except when 



the plate is removed or inserted. H represents the supports for the 



cover C; K K, tapering- sides all round, forming- a funnel-shaped box 



within. 



To prepare the ])late: The cover C being taken out, the cup D is 



charged with a sufficient quantity of iodine, broken into small pieces, 



and covered with the gauze J. The board E is now, with the plate 



attached, placed face downward in its proper j)Osition and the box 



carefully closed. 



In this position the plate remains until the vapor of the iodine has 



produced a definite golden-yellow color — nothing more or less. If 



the operation is prolonged beyond the point at which this effect is 



produced, a violet color is assumed, which is much less sensitive to 



light, and if the yellow coating is too pale the picture produced will 



prove ver_v faint in all its parts. The time for this can not be fixed, 



as it depends entirely on the temperature of the surrounding air. No 



artificial heat must be applied, unless in the case of elevating the 



temperature of an apartment in Avhich the operation may be going on. 



It is also important that the temperature of the inside of the box 



should be the same as it is without, as otherwise a deposition of 



moisture is liable to take place over the surface of the plate. It is 



well to leave a portion of the iodine always in the box, for, as it is 



slowly vaporized, it is absorbed by the wood, and when required it 



is given out over the more extended surface more equally and with 



greater rapidity. 



As, according to the season of the year, the time for producing 

 the required effect may vary from five minutes to half an hour or 

 more, it is necessary, from time to time, to inspect the plate. This 

 is also necessary to see if the iodine is acting eijually on ev^ery part 

 of the silver, as it sometimes happens that the color is sooner pro- 

 duced on one side than on the other, and the plate, when such is the 

 case, nuist be turned one <{uarter round. The plate must be inspected 

 in a darkened room, to which a faint light is admitted in some indi- 

 rect way, as by a door a little open. The board being lifted from the 

 box with both hands, the oi)erator turning the plate toward him 

 ra])idly, observes the color. If too pale, it nnist be returned to the 

 box; but if it has assumed the violet color it is useless, and the whole 

 process nuist be again gone througii. 



From descrii)tion, this operation may appear very difficult, but 

 with u little practice the precise interval necessary to prodiu'e (he 

 best effect is pretty easily guessed at. When th(» proper yellow color 

 is produced, the plate must be put into a frame, which fits the camera 

 obscura, and the doors are instantly closed upon it to prevent the 



